More than half of the THF produced worldwide is manufactured from 1,4-butanediol. It is well-known that THF forms as a result of the dehydration and cyclization of 1,4-butanediol. However, this reaction proceeds very slowly in the absence of a catalyst.
A variety of catalysts are known to promote the conversion of 1,4-butanediol to tetrahydrofuran, including sulfuric acid, oxalic acid, phosphoric acid, copper sulphate, magnesium chloride, zinc chloride, a number of metal oxides and phosphates, sulfonic acid cation exchange resins, acidic nuclear sulfonated cross-linked aromatic hydrocarbon resins, and aluminum hydride-impregnated silica. Processes for the catalytic production of THF from 1,4-butanediol are described in U.S. Pat. Nos. 4,380,657, 4,196,130, 3,726,905 and 3,467,679, the disclosures of which are herein incorporated by reference.
Common disadvantages associated with the catalysts currently used in processes for making THF include limited selectivity, high energy requirements, high cost, corrosiveness, short catalyst lifetime, and/or loss of materials or product.
For example, strong acids, such as sulfuric acid, have the disadvantage of being highly corrosive. This is especially detrimental to metal reaction vessels.
Some other catalysts, such as the cation exchange resins, operate only at very high temperatures. High temperatures increase energy costs as well as promoting the undesirable degradation of the catalyst, the product, and/or the starting material. These resin catalysts often have relatively short lifetimes.
Presently, a common commercial method for THF production involves heating 1,4-butanediol in the presence of sulfuric or phosphoric acid to temperatures exceeding 270.degree. C. at elevated pressures. This process tends to rapidly corrode reaction vessels, and requires a great deal of energy. Under these conditions, a THF product selectivity of 90-99% is usually obtained; a number of by-products form during the reaction. An additional procedure may be needed to remove these by-products.